Firstly, I want to applaud you for seeking to find solutions for your father, he’s lucky to have your care and attention! I’m wondering if you’ve tried to find a naturopathic doctor that may be able to help. I know that many doctors are now doing online consultations.
For some topical applications for the abscess, Mahonia (Oregon grape root) came to mind, it could be applied as a wash, or a decoction of the root could be added to the bath. This herb has strong antibiotic properties. In addition to this, I think calendula could be effective also added to the bathwater as a strong infusion, or applied as a poultice with plant material over a cheesecloth.
I’m also thinking about nourishing, mineral-rich teas that could be supportive – oat, nettles, red clover, in order to replenish the body. I don’t feel equipped to offer more personalized suggestions beyond this, but I really hope you’re able to find a practitioner who can fully support your father.